From the standpoint of energy saving and reduction of environmental burdens, research toward the development of a shortened bake curing step and reduced operation steps has been actively conducted in the field of automotive body coating and industrial product coating.
For example, after electrodeposition coating, intercoating and top coating are typically provided in sequence, and baking has conventionally been performed following each of the intercoating and top coating. However, in an effort to reduce the operation steps in automotive body coating, there has been progress in research aimed at shortening the baking step by performing a single baking step for both the intercoating and top coating after the top coating is provided. This coating method comprising a shortened baking step is becoming the mainstream method.
With respect to shortening the bake curing step, there exists an increasing demand for lowering temperatures and shortening the time required for performing the step.
In the method for forming a multilayer coating film that meets the demand for low-temperature, short-time curing, hydroxyl/isocyanate-crosslinked coating compositions have been considered to be a strong candidate as a coating composition in view of cost efficiency. Hydroxyl/isocyanate-crosslinked coating compositions are obtained by crosslinking hydroxy-containing resins using a polyisocyanate compound as a crosslinking agent and curing the crosslinked resins. Thus, such compositions have been researched so far.
However, the prior art has been unsatisfactory in shortening the curing time in an attempt to improve the curability at low temperatures in a short period of time, and it has also been difficult to satisfy the requirement for an excellent appearance in automobile body coating and other kinds of coating.
Patent Literature 1 discloses a coating composition comprising as essential components a pyrazole-blocked polyisocyanate compound that contains per molecule two or more tertiary isocyanate groups blocked by a pyrazole compound, and a hydroxy-containing resin having a specific hydroxyl value and a specific weight average molecular weight. Patent Literature 1 also discloses a method for forming a coating film using the coating composition. However, the method for forming a coating film using such a coating composition has been unsatisfactory in curability under short-time curing conditions.
Patent Literature 2 discloses a curable coating composition comprising a resin composition for preparing a coating composition and a (blocked) polyisocyanate compound, wherein the resin composition comprises a polyol (A1), a polyether polyol (B), and a solvent (C), wherein the polyol (A1) has a ring structure. Patent Literature 2 further discloses another curable coating composition comprising an organometallic compound and an acidic substance in addition to the aforementioned curable coating composition.
This coating composition is excellent in curability at low temperatures in a short period of time in an embodiment where the coating composition comprises an organometallic compound and an acidic substance; however, the pot life is unsatisfactory. The finished appearance of a multilayer coating film obtained through the method for forming a coating film using such a coating composition is also sometimes unsatisfactory.
Patent Literature 3 discloses a method for forming on a rigid resin part a coating film excellent in finished appearance, coating film performance, coating composition stability, and the like, through baking at low temperatures in a short period of time. The method comprises the steps of applying a colored base coating composition and subsequently a clear coating composition to a rigid resin part, followed by curing of both coating films, characterized in that the clear coating composition contains in a specific ratio: an acrylic resin obtained by copolymerizing a monomer mixture having a specific composition; and a curing agent.
However, the method for forming a coating film is sometimes unsatisfactory in curability under short-time curing conditions, and the resulting multilayer coating film can also be poor in finished appearance.